Bolduc David M, Montagna Daniel R, Gu Yongli, Selkoe Dennis J, Wolfe Michael S
Ann Romney Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115.
Ann Romney Center for Neurologic Diseases, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115
Proc Natl Acad Sci U S A. 2016 Feb 2;113(5):E509-18. doi: 10.1073/pnas.1512952113. Epub 2015 Dec 22.
γ-Secretase is an intramembrane-cleaving protease that processes many type-I integral membrane proteins within the lipid bilayer, an event preceded by shedding of most of the substrate's ectodomain by α- or β-secretases. The mechanism by which γ-secretase selectively recognizes and recruits ectodomain-shed substrates for catalysis remains unclear. In contrast to previous reports that substrate is actively recruited for catalysis when its remaining short ectodomain interacts with the nicastrin component of γ-secretase, we find that substrate ectodomain is entirely dispensable for cleavage. Instead, γ-secretase-substrate binding is driven by an apparent tight-binding interaction derived from substrate transmembrane domain, a mechanism in stark contrast to rhomboid--another family of intramembrane-cleaving proteases. Disruption of the nicastrin fold allows for more efficient cleavage of substrates retaining longer ectodomains, indicating that nicastrin actively excludes larger substrates through steric hindrance, thus serving as a molecular gatekeeper for substrate binding and catalysis.
γ-分泌酶是一种膜内裂解蛋白酶，可在脂质双层中加工许多I型整合膜蛋白，此过程之前，大多数底物的胞外域会被α-或β-分泌酶切割。γ-分泌酶选择性识别并募集胞外域已被切割的底物进行催化的机制仍不清楚。与之前的报道相反，之前的报道称当底物剩余的短胞外域与γ-分泌酶的尼卡斯特林组分相互作用时，底物会被主动募集进行催化，而我们发现底物胞外域对于切割完全是可有可无的。相反，γ-分泌酶与底物的结合是由底物跨膜域产生的明显紧密结合相互作用驱动的，这一机制与另一家族的膜内裂解蛋白酶——类菱形蛋白酶形成鲜明对比。尼卡斯特林折叠结构的破坏能使保留较长胞外域的底物更有效地被切割，这表明尼卡斯特林通过空间位阻主动排除较大的底物，从而作为底物结合和催化的分子守门人。
